MgO-supported gold prepared by adsorption of Au(CH3)(2)(acac) (acac is acetylacetonate) on partially dehydroxylated MgO was activated for CO oxidation catalysis by treatment in flowing helium at 473 K. X-ray absorption spectra showed that the activation involved reduction of the gold and formation of clusters (with an average diameter < 10 angstrom) in which the gold was essentially zerovalent. During CO oxidation catalysis in a batch reactor, at least some of the gold was oxidized. as evidenced by the appearance of an Au delta+-CO band at 2151 cm(-1) in the infrared (IR) spectrum. During operation in a flow reactor, the catalyst underwent deactivation, accumulating species such as carbonate and bicarbonate on its surface, as indicated by IR spectra. The accumulation of such species on the MgO support took place only during the initial period of operation of the catalyst, whereas the accumulation of such species on the gold continued throughout the operation, consistent with the inference that these species blocked catalytically active sites on the gold. The catalyst was reactivated by decomposition of these species by treatment in helium at 473 K. After three activation-deactivation cycles, the average diameter of the supported gold clusters had increased to about 30 angstrom, and the catalytic activity increased. Thus, the results provide a resolution of the separate effects on the catalytic activity of gold aggregation and accumulation of species such as carbonates and bicarbonates. (C) 2008 Elsevier Inc. All rights reserved.